The present invention relates to a method and a mechanism for recycling  transporting toner in an electrophotographic printer.
An electrophotographic printer forms a lateral electrostatic image on a photosensitive drum, develops the image by application of toner, then transfers the toner from the drum to a printing medium such as a sheet of paper. In many electrophotographic printers, the photosensitive drum and developing unit are part of a replaceable cartridge that also includes a cleaning unit for cleaning the photosensitive drum by removing toner that fails to be transferred to the printing medium.
Many electrophotographic printers have a recycling mechanism that returns toner form the cleaning unit to the developing unit, so that toner recovered by the cleaning unit can be reused. Such recycling of toner has several advantages: more pages can be printed before the toner supply has to be replenished; no tank is needed to store toner removed by the cleaning unit; and the toner removed by the cleaning unit does not have to be disposed of.
The toner recycling mechanism is part of the replaceable cartridge. A conventional toner recycling mechanism comprises three helical screw conveyors. The first screw conveyor conveys toner to one end of the cleaning unit. The second screw conveyor conveys toner from this end of the cleaning unit to the corresponding end of the developing unit. The third screw conveyor conveys toner into the developing unit.
The replaceable cartridge also includes various rollers, such as a developing roller for applying toner to the photosensitive drum and a supply roller for supplying toner to the developing roller. The first and third screw conveyors of the toner recycling mechanism are oriented parallel to these rollers, but the second screw conveyor is oriented at a right angle to the rollers. To avoid interference with the rollers, the second screw conveyor must be disposed at one side of the cartridge, beyond the ends of the roller shafts. The cartridge must accordingly be widened to accommodate the second screw conveyor, but this undesirably increase the dimensions of the printer.
Another disadvantage of the conventional toner recycling mechanism is that the second screw conveyor must be driven by a gear train including a worm gear. This type of gear train produces a thrust force that places an extra load on the driving motor, leading to uneven motion and irregular printing. The existence of the worm gear train also makes the replaceable cartridge more difficult to assemble.